desktop: tray menu panels and pinnable context menus

The tray's right-click menu is drawn by the app: one tree from tray.rs
(stable submenu keys), one frameless always-on-top menu-N window per
panel rendered by tray-menu.html with the design system, placed inside
the monitor's work area. A pure panel book keeps transient and pinned
panels apart; pinned panels (pin button, Ctrl+click, middle click) are
draggable, step aside from each other and are saved in state.json
(menuPins) and restored at start. tray_update pushes the tree to the
open panels, which update in place; closed panels are destroyed. The
native menu, rendered from the same tree, stays as the fallback and on
Linux. Only menu-* windows get the menu_* commands (menu.json).

In-app context menus get the same behaviour through a pure menu stack
(ui/menu-stack.ts): pin a menu, tear off a submenu, pinned panels
survive the next menu, are draggable and re-read their entries from
live sources so data changes update them in place.

Also drops a stray diff3 marker line from CHANGELOG.md.

apps/desktop/src-tauri/build.rs

@@ -5,7 +5,7 @@ include!("src/commands_list.rs");
 fn main() {
     tauri_build::try_build(
         tauri_build::Attributes::new()
-            .app_manifest(tauri_build::AppManifest::new().commands(Box::leak([COMMANDS, CARD_COMMANDS].concat().into_boxed_slice()))),
+            .app_manifest(tauri_build::AppManifest::new().commands(Box::leak([COMMANDS, CARD_COMMANDS, MENU_COMMANDS].concat().into_boxed_slice()))),
     )
     .expect("tauri build script failed");
 }